[Object/ELF] - Fix a position calculation expression in ELFFile<ELFT>::getEntry().

It fixes now what 1c991f90 tried to fix.
(A test case failture on 32-bit Arch Linux)

On 32-bit hosts it still fails (because it truncates the `Pos` value to 32 bits).
It seems happens because of `sizeof` that returns `size_t`, which has a
different size on 32/64 bits hosts.

I've tested on a 32-bit host and verified that relocation-errors.test test and
other LLVM tools tests pass now.